Differences of AC Vs. DC Motors

Written by j.t. barett
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Differences of AC Vs. DC Motors
Small appliances and toys use compact DC motors. (Jupiterimages/Pixland/Getty Images)

While alternating current (AC) and direct current (DC) electric motors have features in common, their designs and uses differ. The AC motor's simpler design makes it more efficient but it has a restricted range of speeds. Toys, electric razors and other gadgets needing small lightweight motors go with DC, and larger appliances like vacuum cleaners use AC motors.

Power Source

The direct current that powers DC motors typically comes from batteries. DC electricity flows in one direction, from the positive to the negative terminal of the source. You can reverse the motor's direction by simply reversing its positive and negative connections. An AC source, as from a household outlet, has a wavelike electrical flow that reverses direction 120 times per second. The AC motor's design takes advantage of the current's changing direction to make the motor spin.

Durability and Efficiency

In a DC motor, current flows through a pair of conductive brushes that touch a set of split contacts on a rotor. The split contacts serve as a way to reverse the polarity of a electromagnet coils. The coils repel against permanent magnets in the motor housing and make the rotor spin. Friction from the brushes rubbing on the contacts saps the motor's power and eventually wears the brushes out. AC motors run on an induction principle that eliminates the need for brushes, making them more efficient and durable.

Speed Control

You can vary the speed of a DC motor by changing its supply voltage. For example, you can make a 3-volt motor run twice as fast by running it at 6 volts. AC motors, on the other hand, run at speeds fixed to the 60hz AC line frequency. Clever designers use pulley and gear systems to produce different speeds and reverse the motor's spin. Sophisticated electronic controls can, in effect, vary the AC line frequency, though this requires a specially built motor.

Starting Torque

A motor's starting torque is the twisting force it can muster from a dead stop. DC motors have decent starting torque, able to start even when mated to a heavy load. If you have the luxury of three-phase industrial AC power, an AC motor has strong starting torque. However, most homes have single-phase AC, which puts AC motors at a disadvantage. Starting capacitors solve this problem, though this adds to the motor's cost.

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